How to create Mapping rules for custom entities in EF6?

Yes, there is other way to map your classes and it better option. At least, I thonk so. You can create mapper for your model which inherits generic EntityTypeConfiguration and add this mapper OnModelCreating. This way your code will stay clean and its much more easier to manage mappings if you have a lot of models.

Model class:

public class Person
{
    public int Id { get; set; }
    public string FullName { get; set; }
    public int Age { get; set; }
}

Mapper class:

internal class PersonMap
    : EntityTypeConfiguration<Person>
{
    public PersonMap()
    {
        // Primary key
        this.HasKey(m => m.Id);

        // Properties
        this.Property(m => m.FullName)
            .HasMaxLength(50);      

        // Table & column mappings
        this.ToTable("TABLE_NAME", "SCHEMA_NAME")
        this.Property(m => m.Id).HasColumnName("ID");
        this.Property(m => m.FullName).HasColumnName("FULL_NAME");
        this.Property(m => m.Age).HasColumnName("AGE");

        // Relationship mappings
        // Map your naviagion properties here if you have any.
    }
}

Then you add mapper at OnModelCreating method:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Configurations.Add(new PersonMap());

    base.OnModelCreating(modelBuilder);
}